Visual Communication

Many insects communicate with visual signals. The color patterns and other markings on the wings of butterflies and moths facilitate species recognition in much the same way colored uniforms reveal the players' affiliations on a football field. Some insects use bright colors, eyespots, or other distinctive patterns to scare away predators, to advertise their ability to sting, or to mimic the appearance of another unpalatable species. Other insects use dance-like body movements to attract a mate or to communicate with nestmates. Most of these signals are effective only as long as they are visible in daylight. But a few insects (fireflies, for example) can generate their own light and use visual signals that can be seen at night.

Pros and Cons of Visual Communication

Advantages:

Effective over long distances

Can be used while moving

Fast -- speed of light

Effective in all directions (independent of wind)

Passive signals require no expenditure of energy

Disadvantages:

Requires a clear line of sight

Visual signals may be intercepted by predators

Only effective in daylight (in fireflies, only at night)

Active signals may be metabolically "expensive" to produce

Passive signals, such as eyespots and color patterns can serve as a form of "free advertising". The colorful wings of a butterfly, for example, are a "billboard" publicizing its species identity. Individual insects incur little or no metabolic cost for displaying these messages because they are an integral part of the integument. It may be prudent to hide these signals from a potential predator, so some insects have a way to conceal their message when necessary. The red admiral butterfly, for example, has bright, distinctive markings on the upper wing surface and drab, protective coloration on the underside.

Active signals, like body movements and light flashes, are more costly to produce, but they can be withheld from use at inappropriate times. They may also have a higher information content because signal frequency, duration, or periodicity may convey additional meaning. In fireflies, for example, pulses of light are used in a courtship dialogue between a male (usually flying) and a female (usually perched in the vegetation). Each species has a unique flash pattern and response time. Males of Photinus pyralis emit a single "J"-shape flash during a rising flight movement. A female responds with a single flash after a two second interval. In Photinus consumilis, males emit a series of 3-5 short flashes and the females respond with a double flash. Roles are reversed in some tropical species where the females fly and the males signal from perches in the vegetation.

Ultraviolet reflection from a male
alfalfa butterfly in simulated flight

Unlike humans, many insects have the ability to see ultraviolet light. Not surprisingly, some species communicate using wavelengths in this part of the spectrum. Female cabbage butterflies, for example, have ultraviolet-reflecting scales on the dorsal wing surface. When they fly, each downstroke of the wing creates a brief "flash" of UV that males apparently recognize as the flight signature of a potential mate. A "flashing female" may attract several males who engage in aerial courship displays. In alfalfa butterflies, only males have the UV-reflective scales. They flutter in front of the females to create a flickering courtship display. Missing scales reduce the wings' reflectivity -- a sign of aging that impairs a male's ability to seduce a mate.